1. Field of the Invention
The present invention relates to a soldering method and soldering apparatus for dropping molten or semi-molten solder (hereinafter referred to simply as molten solder) on a workpiece to effect soldering.
2. Prior Art Statement
Japanese Patent Publication No. 34209/1971 discloses a soldering apparatus for applying molten solder onto a desired point or area of a workpiece, such as a printed circuit board, wherein a conical pot is used for melting a solid solder. A wire solder is cut and fed to the conical pot to be melted therein and the molten solder is allowed to drop through a hole provided at the bottom of the pot. However, this type of soldering apparatus has a problem that some portion of the solid solder drops through the hole in the unmelted state while the other portion of the solder is dropped in the overheated condition, since it is impossible to retain the whole solid solder for a constant time, resulting in uneven and unreliable soldering.
Unexamined Japanese Utility Model Publication No. 77427/1975 discloses a soldering apparatus having a conical melting pot provided with a needle valve at the bottom of the pot. In this know apparatus, a molten mass of solder is contained in the conical melting pot, and a constant volume of the molten solder is allowed to drop through the needle valve. However, due to surface tension of the molten solder, it becomes impossible to control the volume of dropping molten solder precisely particularly when the volume of molten solder to be applied onto the workpiece is relatively small, as is the case where the workpiece is a printed circuit board.
In order to solve the aforementioned problems of the known apparatuses, the inventor of this invention has previously proposed an improvement in soldering in my earlier Japanese Patent Application No. 110505/1986 which has been published as JP-A-62-270272. According to my previous proposal, a solder melting pot is formed by plural trowel members which engage with each other to hold a solid solder tip or piece for a pre-set time to melt the same and then they are separated to allow the molten solder to drop onto a desired point or area of a workpiece.
However, in the method according to my prior proposal, the time for falling the solid solder piece from the solder cutter to the solder melting pot cannot be controlled so that it becomes hard to control the temperature of the molten solder in the solder melting pot to a constant temperature.
To solve the aforementioned problem, the inventor of this application has proposed another method in which a solder piece cut from a wire solder to have a constant length (i.e. constant volume) is tentatively received in a cradle to hold the same in the stationary condition, and then the cradle is inverted to allow the solder piece to drop through a tubular solder piece guide into a solder melting pot (Unexamined Japanese Patent Publication No. 2-182373, which corresponds to U.S. Pat. No. 5,038,995 and EP-0378369B).
However, even in the method according to my later proposal, there is a problem that the time cost for dropping the solder piece cannot be controlled to a constant time since the solder piece dropping from the cradle often contacts with the interior wall of the solder piece guide to change the dropping condition. Since the timing from which the solder piece begins to drop is detected and it is estimated that the solder piece is melted in the solder melting pot after the lapse of a constant time duration from that timing, the heating time for melting the solder piece is differentiated from the thus estimated time when the time cost for dropping the solder piece from the cradle to the solder melting pot is changed. This leads to temperature fluctuation of the molten solder from the optimum temperature so that false soldering, such as knotty or tubercled soldering, is resulted when the temperature of molten solder is lower than the optimum temperature or the molten solder is deteriorated in quantity when the temperature thereof is higher than the optimum temperature.
To solve the aforesaid problem, the inventor of this application has proposed a further method wherein melting of a solder piece in the solder melting pot is detected by sensing, for example, the light reflected by the surface of molten solder mass, and wherein the time duration for melting the solder and the temperature of molten solder are controlled (Unexamined Japanese Patent Publication No. 5-343842, which corresponds to U.S. Pat. No. 5,297,718 and EP-0574218A).
According to the above-mentioned practice, a wire solder is cut to make the solid solder piece having a predetermined length, and thus solid solder piece is fed to the solder melting point. However, this practice involves a problem that particularly when a flux coated or cored wire solder is cut, the residing flux sticks to the surrounding portions of a solder cutter and a solder feeder to prevent a smooth feed of the solid solder piece. There is also a problem that smoke generated from molten solder and flux deteriorates the sensitivity of a photosensor.